1. Field of the Invention
The present invention relates to a thermal transfer type image forming device for forming an image using hot melt ink.
2. Description of Related Art
As shown in FIG. 1, a conventional thermal transfer type image forming device 401 includes a thermal head unit 410, an ink supply medium 420 serving as an ink carrying member, and a thermal transfer mechanism 430. The thermal head unit 410 includes a head 411 and a driving source 412. Although not shown in the drawings, the head 411 has a plurality of heating elements each connected to the driving source 412. The driving source outputs driving signals to the heating elements based on image signals transmitted from a control circuit. Upon receiving the driving signals, the heating elements selectively generate heat. The ink supply medium 420 has a base film 412 and a hot melt ink layer 422 formed on the base film 412. The thermal-transfer mechanism 430 has a platen roller 432 positioned in confrontation with the head 411 with the ink supply medium 420 and a recording medium 431 sandwiched therebetween. By being selectively driven, the heating elements thermally transfer the ink from the hot melt ink layer 422 onto the recording medium 431. That is, heat generated by the driven heating elements melts the ink in the hot melt ink layer 422. The melted ink is then supplied onto the recording medium 431, thereby forming an image on the recording medium 431.
Thermal transfer of ink onto the recording medium 431 as shown in FIG. 1 forms ink voids 422a and ink regions 422b in the ink layer 422 of the ink supply medium 420. Therefore, the ink supply medium 420 can be used only once. More specifically, because the heating elements of the head 411 are selectively driven to thermally transfer ink at only selected positions of the ink layer 422, the ink at only the selected positions is transferred onto the recording medium 430. As a result, almost no ink is left on the base film 421 at the selected positions. These selected positions correspond to the ink voids 422a. Although ink remains at the ink regions 422b on the base film 421 at unselected positions, the ink supply medium 420 cannot be reused because of the voids 422a. The ink supply medium 420 is disposed with after only a single use, resulting in wasting a large amount of ink and increasing running costs.
In order to overcome this problem, Japanese Patent-Application Publication (Kokai) (hereinafter referred to as "JP") No. HEI-5-238028 described an image forming device in which an ink carrying member is recovered after used. The image forming device includes an ink tank containing ink. Ink in the ink tank is kept in its melted state by a heater. The melted ink is supplied from a cylindrically-shaped ink supply portion onto an ink support film serving as an ink carrying member. However, a great amount of energy is required for maintaining ink in its melted state in the ink tank. This increases the running cost.
Also, JP No. HEI-4-126283 describes an image forming device having an ink carrying member capable of being used repeatedly. The ink carrying member is a thermal-transfer sheet made from a foamed resin which is holding ink. Because ink is oozed out to a surface of the sheet by a recording head as needed, the ink carrying member can be used repeatedly without having voids. However, the ink carrying member is not durable for a long period of time because the resin containing ink may be easily degraded by being subjected to heat during repeated thermal transfer operation. Also, because the resin has poor heat conducting properties, its temperature increases and decreases at a relatively slow rate. This limits the speed of printing operations.
In U.S. Pat. No. 5,708,468, the present applicant has proposed a thermal transfer type image forming device 501 shown in FIG. 2. Ink melted from a hot melt ink member 510 by a heater 520 is supplied onto an ink retaining roller 530. A peripheral surface of the ink retaining roller 530 is made of a foamed resin in which the ink is held. When the ink is brought into a confrontation with a thermal head 550 as the ink retaining roller 530 rotates, the thermal head 550 selectively generates heat to melt the ink so that the melted ink is transferred onto a recording medium 540 positioned between the ink retaining roller 530 and the thermal head 550. After the ink is transferred onto the recording medium 540, the ink retaining roller 530 is resupplied with ink. In this way, the ink retaining roller 530 is repeatedly used.
As shown in FIG. 2, the thermal head 550 is disposed such that the recording medium 540 is interposed between the thermal head 550 and the ink retaining roller 530. Because heat generated by the thermal head 550 is supplied to the ink on the ink retaining roller 530 from a side close to the recording medium 540, only the ink held close to the recording medium 540 can be effectively transferred onto the recording medium 540.
However, heat from the thermal head 550 may not be supplied to the ink because of a thickness of the recording medium 540 or a material forming the recording medium 540. In this case, the ink will not be transferred onto the recording medium 540.
There has been also proposed a tandem type image forming device including a plurality of image forming units and an intermediate transfer body. Each image forming unit transfers one of different colored inks onto the intermediate transfer body. The inks from the image forming units collectively form a multicolored image. That is, different colored images are formed in an overlapping relation by the image forming units so as to form a single multicolored image.
This type of image forming device can form a multicolored image in a relatively short time, and it is necessary for each image forming unit to operate in synchronization and transfer ink in a uniform time duration.
However, in a thermal transfer tandem type image forming device, when one of the image forming units thermally transfers ink onto an existing ink image, the ink in the existing image is also heated. This may melt the ink of the existing image also, and disturb and blur the overall image.
In order to overcome these problems, JP No. HEI-4-41284 proposed to use different colored inks having different melting points. However, each colored ink with different melting point takes a different time duration to be thermally transferred. Because, in a tandem type image forming device, it is necessary for each image forming unit to operate in synchronization and transfer ink in a uniform time duration as described above, it has been difficult to configure a tandem type image forming device using different colored inks each having a different melting point.
Further, there has been known an image forming device including a laser unit for emitting laser beams and an ink carrying member having an ink layer formed on a transparent substrate. Laser beams are selectively irradiated onto designated spots on the ink holding member so that ink at the spots is thermally transferred onto a recording medium. Because the laser beam can be irradiated on an extremely small spot, an image with high resolution can be obtained.
However, the laser unit outputs only a small amount of heat compared to heat energy required to thermally transfer hot melt ink. Therefore, it takes a relatively long time for the laser unit to melt the hot melt ink. In order to overcome this problem, Japanese Patent-Application Publication (Kokoku) No. HEI-1-21789 proposed an image forming device having a preheating unit for preheating an ink layer of an ink carrying member. A control mechanism controls the amount of heat generated by the preheating unit in accordance with a detected temperature of the ink layer. With this configuration, the laser unit requires less energy, that is, less time, to melt the preheated ink.
However, the additional components, that is, the preheating unit and the control mechanism, increase the size of the image forming device and complicate its structure, resulting in increasing manufacturing costs of the device.